System and a method of analyzing and monitoring interfering movements of an inertial unit during a stage of static alignment

ABSTRACT

A system and to a method of analyzing and monitoring interfering movements of an inertial unit of an aircraft during a stage of statically aligning the inertial unit. During the static alignment stage, measurements of the velocity of the aircraft relative to the ground are acquired, and states of a mirror process having a model that is close to the model of the process of aligning the inertial unit are estimated. The states of the mirror process are estimated from observations constituted by the measurements of velocity relative to the ground. Finally, the estimates of the states are compared with respective validation thresholds in order to validate or not validate said alignment of the inertial unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/945,525, filed Apr. 4, 2018, which claims priority to French patentapplication No. FR 1770355 filed on Apr. 7, 2017, the disclosures ofwhich are incorporated in their entirety by reference herein.

BACKGROUND OF THE INVENTION (1) Field of the Invention

The general field of the present invention is that of sensor systems foraircraft, and in particular inertial units.

The present invention relates to a system and to a method of analyzingand monitoring interfering movements of an inertial unit during a stageof static alignment of the inertial unit.

More precisely, the present invention makes it possible to estimateorientation errors and velocity errors affecting the measurements of theinertial unit as a result of movements during its static initializationstage during which:

-   -   the inertial unit aligns its axes on geographical axes; and    -   the inertial unit zeros its velocity estimate.

2) Description of Related Art

An inertial unit is an instrument that is used in particular in thefield of aviation, but that can be installed on board any type ofvehicle, specifically a ship, a submarine, an aircraft, a missile, orindeed a spacecraft. An inertial unit is capable of integrating themovements to which it is subjected, and in particular accelerations andangular velocities, in order to provide estimates of the orientation, ofthe linear velocity, and also of the position of the vehicle. By way ofexample, the orientation of a vehicle may be defined by roll, pitching,and heading angles.

An inertial unit generally has six sensors, namely three gyros formeasuring angular velocities about three axes, and three accelerometersarranged to measure accelerations along those three axes. An inertialunit has no need for any external information. An inertial unit makesuse exclusively of measurements supplied by its specific internalsensors for sensing angular velocities and forces in order to estimateits orientation, its velocity, and its position by integrating themeasurements from its internal sensors over time.

Because of the time integration process that is used, it is essential toperform a stage of initializing the inertial unit whenever it isstarted. The initialization stage is generally performed when thevehicle that uses the inertial unit is stationary, typically prior totakeoff of an aircraft.

An initialization stage comprises in particular an alignment stageduring which the following are estimated:

a vertical direction from the measurements of the accelerometers thatserve to determine the direction of terrestrial gravity, and thus of thevertical;

a geographical North direction using measurements of the gyros thatdetect the terrestrial rotation vector and thus, by projecting thisvector onto the horizontal plane, the North direction; and

a velocity vector relative to the ground.

Usually, an initial position is not estimated by the inertial unit. Onthe contrary, it needs to be input, e.g. by the crew.

An alignment process that is typically used in an inertial unit seeks tocause certain values to converge on zero, the values being firstly oftwo angles of inclination of a virtual platform as calculated by theinertial unit, and secondly an angle of misalignment about the verticaldirection of the virtual platform. This alignment process typicallymakes use of a Kalman filter.

Certain alignment processes are based on the assumption that the vehicleis stationary and they therefore also seek to set to zero the twohorizontal components of the velocity of the vehicle relative to theground. This is referred to as “static” alignment. Below in thisdocument, consideration is given only to the static alignment situation.Thus, in order to lighten the text, the adjective “static” is oftenomitted, however on each occurrence of the term “alignment”, it shouldbe understood as “static alignment”.

However, while the alignment process is taking place, the aircraft mightbe subjected to interfering movements, such as being towed by a tractorvehicle. The assumption that the vehicle is stationary is then wrong.This gives rise to poor accuracy of the inertial unit. The inaccuraciesthat result from interfering movements during alignment can reach valuesthat are sufficient to subsequently compromise the safety of the flightduring a subsequent stage of navigation while flying. In order to avoidpolluting the inertial unit, various protective measures are known.

In order to monitor whether the aircraft is indeed stationary during thestage of aligning its inertial unit, it is known by way of example toanalyze the amplitude of the residue of the Kalman estimator that isperforming the alignment.

That type of monitoring by means of the alignment process of an inertialunit is satisfactory for detecting interfering movements of relativelyhigh frequency, e.g. caused by the aircraft being towed.

Document EP 2 488 829 describes a method of that type. That documentdescribes a method of detecting interfering movements based on comparinga residual signal with a predefined threshold, the residual signal beingthe residue between a raw position signal obtained by integratingsignals from the sensors of an inertial unit and a theoretical signalmodeling the raw position signal as a function of a predetermined modelfor error in the absence of movement. According to that document,movements of large amplitude, whether of short or long duration, can bedetected by comparing acceleration measurements with thresholds, whereasmovements of small amplitude and short duration can be detected from theparameters of a Kalman filter.

Also known is Document FR 2 940 427 which describes the use of aninertial unit for determining a heading using two different modes:namely a “North-seeking” mode, with the inertial unit being fixedrelative to the ground and horizontal, and a free gyro mode with theinertial unit then being stationary relative to the ground. A differencebetween the headings obtained using those two modes is determined, andas a function of that difference, the heading that is obtained by usingone or the other of the modes is retained for use.

Finally, Document US 2006/047427 describes a system and a method foraligning an inertial unit that can be used even when the aircraft towhich it is fitted is moving. That system has an inertial unit supplyingpurely inertial navigation information, and an external source, such asa global navigation satellite system (GNSS) receiver, that isindependent from the inertial unit and that also supplies navigationinformation. The system has navigation logic receiving the navigationinformation and provided with recursive filters in order to process thenavigation information, and also logic for verifying integrity in orderto monitor, compare, and combine those two sources of navigationinformation. In particular, errors between those two sources ofnavigation information can be defined and compared with predeterminedthresholds in order to define what navigation information supplied bythe inertial unit and/or the external source can be used for navigationpurposes.

Nevertheless, such alignment processes present the drawback of not beingeffective for detecting certain specific interfering movements, inparticular slow movements having a period that coincides with theduration of the entire alignment stage.

This problem applies particularly to a rotary wing aircraft taking offfrom an oil platform at sea and potentially being subjected thereon tomovements that are very slow through an amplitude of several meters,e.g. due to the system for stabilizing the position of the oil platform.

It can thus be seen that inertial units and the static alignmentprocesses they implement include devices for detecting interferingmovements during the alignment stage. However, those detection devicesare not capable of exhaustively detecting all types of interferingmovements that might disturb the alignment stage, and consequently theygive rise to dangerous errors in the data supplied by the inertial unitduring a subsequent stage of navigation in flight.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is thus to be unaffected by theabove-mentioned limitations so as to be able to detect any type ofinterfering movements to which an inertial unit might be subjected andthat could disturb the static alignment stage. Thereafter, the presentinvention can inhibit validating the static alignment stage in order toavoid the inertial unit supplying data that is erroneous. Alternatively,after detecting interfering movements and making a quantified estimateof their effects on the inertial unit, the present invention can correctthe erroneous data supplied by the inertial unit in order to make thatdata usable, i.e. sufficiently accurate.

In this context, the present invention provides a method of analyzingand monitoring interfering movements of an inertial unit of an aircraftduring a stage of statically aligning the inertial unit.

The method of the invention is remarkable in that during the stage ofstatically aligning the inertial unit, the method performs the followingtwo steps:

an acquisition step for acquiring measurements of the movement of theaircraft relative to the ground; and

an estimation step for estimating states of a mirror process having amodel close to the model of the process of aligning the inertial unit,the states of the mirror process being estimated from observationsconstituted by the movement measurements.

This method is intended more particularly for use on board an aircraft,however it may be applied to any vehicle that uses an inertial unit.

In order to apply the method, the aircraft has at least one inertialunit, together with a movement sensor making it possible to measure themovements of the aircraft relative to the ground, and an estimator forestimating a mirror process of structure close to the structure of theprocess of aligning the inertial unit. This estimator for estimating amirror process close to the process of aligning the inertial unit isconfigured to perform the method of analyzing and monitoring interferingmovements of an inertial unit during an alignment stage.

In this context, when a process or a process model is said to be “closeto” process models or processes that are nearly identical, that means amodel of a mirror process may in particular be simplified or elseapproximated, or indeed both simplified and approximated relative to themodel of the alignment process. For example, the model of a mirrorprocess is defined by a set of simplified differential equations, of adegree that is lower than or equal to the number of states of the modelof the alignment process.

By way of example, the movement sensor is a velocity sensor supplyingmeasurements of a velocity {right arrow over (v_(g))} of the aircraftrelative to the ground. The movement sensor may equally well be aposition sensor providing measurements of the position {right arrow over(x_(g))} of the aircraft relative to the ground. In both situations, themovement sensor may be a GNSS receiver or a Doppler effect radar, forexample.

As mentioned above, and by way of example, a process of aligning aninertial unit may be based on a model having one or more states, inparticular states representing orientation and velocity errors, withvariation thereof being governed by one or more differential equationsthat, on being solved during the alignment stage, make it possible tocause the estimated states of the model to converge towards zero values.The alignment process can thus be in the form of an estimator forestimating orientation and velocity errors.

The estimator for estimating orientation and velocity errors isgenerally associated with a process for detecting interfering movements,which process is typically based on analyzing residues.

In spite of the devices for detecting interfering movements installed inthe inertial unit, possibly integrated in the alignment process, someinterfering movements of the inertial unit remain undetectable whilenevertheless giving rise to disturbances. Under such circumstances, thealignment of an inertial unit in the prior art might be considered asbeing valid even though the data then being supplied by the inertialunit is severely erroneous and contains orientation and velocity errorsexceeding the acceptable maximum.

Interfering movements that are undetectable independently by the processfor detecting interfering movements of the inertial unit are movementsthat vary in a manner that is identical or close to variation in thestate of the generator process as constituted by the differentialequation solver of the model of the process for aligning the inertialunit.

Specifically, when such movements match the variation in those states,the differences between the characteristics of the movements and thestates as estimated by the alignment process are constantlysubstantially zero. Consequently, the alignment process behaves asthough the aircraft were stationary in spite of the presence of thosemovements.

Under such circumstances, any movement of the inertial unit having atime function that can be generated by the differential equation solverof the alignment process, and regardless of its amplitude, cannot bedistinguished from being stationary by the inertial sensors on theirown, and gives rise to inertial estimates of orientation and velocitythat are erroneous at the end of aligning the inertial unit, and thatconsequently continue to be erroneous throughout the stage of navigationthat follows the stage of alignment. The amplitude of such errors isunbounded.

In order to remedy that problem, the method of the inventionadvantageously makes use of a mirror process having a model that isclose to the model for error propagation in the process of aligning theinertial unit. The method of the invention then makes it possible toestimate states of the mirror process from observations constituted bythe movement measurements. These estimates of states constituteestimates of orientation and velocity errors affecting the data suppliedby the inertial unit at the end of the alignment stage.

The model of this mirror process is preferably identical to the model ofthe process of aligning the inertial unit. The differential equations ofgoverning variation in the state of the model of the mirror process andof the model of the alignment process are then the same, and inparticular both models have the same number of states.

Nevertheless, it is also possible to simplify the mirror processcompared with the alignment process without going beyond the ambit ofthe invention. In particular, certain states of the alignment processmay have very little impact on the overall behavior of the model and cantherefore be ignored and need not be incorporated in the mirror process.

For example, when the movement measurements are measurements of thevelocity {right arrow over (v_(g))} of the aircraft relative to theground, the mirror process typically has five states, namely three errorangles for the orientation of the virtual platform of the inertial unitrelative to the geographical axes, and two horizontal components ofvelocity relative to ground. The two outputs of the mirror process areestimates of these horizontal components of velocity relative to theground.

In a particular implementation of the invention, the mirror process isused in an open loop. These two outputs of the mirror process then varyin compliance with polynomial functions of time, firstly a second degreepolynomial function for the North/South component of the velocity, andsecondly a first degree polynomial function for the East/West componentof the velocity. In this particular implementation, the estimation bythe mirror process consists in calculating:

-   -   i) a parabola corresponding to the second degree polynomial        function that is the closest to the North/South components of        the measurements of velocity relative to the ground; and    -   ii) a straight line, corresponding to the first degree        polynomial function that is the closest to the East/West        components of the measurements of the velocity relative to the        ground.

This calculation of the closest polynomial functions may be performed bythe least squares method, which may be recursive or non-recursive.

In another implementation of the invention, and by way of example, thestates the mirror process may be estimated by Kalman filtering in whichthe states are those of the mirror process and for which theobservations are the measurements of movement relative to the ground.

When using the non-recursive least squares method, measurements of themovement of the aircraft estimates are acquired and states of the mirrorprocess are estimated sequentially. When using the recursive leastsquares methods or a Kalman filter, the measurements of the movement ofthe aircraft are acquired and the states of the mirror process areestimated simultaneously.

Furthermore, in variants, the method of the invention may include anadditional step.

Thus, in a first variant, the additional step of the method isadvantageously comparing the absolute value of at least one estimate ofone of the states with at least one validation threshold in order tovalidate or not validate the alignment of the inertial unit.

For example, in this first variant, thresholds are defined forvalidating the alignment. These validation thresholds are generallydefined during a stage of developing the system and the method ofanalyzing and monitoring of the invention. The values of thesevalidation thresholds are selected as a function of the accuracyexpected from the inertial unit. Each validation threshold correspondsto a respective one of the estimated states, these estimated statescorresponding respectively to orientation and velocity errors due to theinterfering movements during the alignment stage and affecting the datasupplied by the inertial unit after the alignment stage.

As a result, during the comparison, the absolute value of at least oneestimate of the state is compared with the corresponding validationthreshold, and an “alignment invalid” signal is activated whenever theabsolute value of an estimate of a state is greater than thecorresponding validation threshold. The method may advantageously makeit possible to limit the comparison to estimates of states having thegreatest influence on the performance of the inertial unit.

Preferably, the individual absolute values of the estimates of thestates are compared with respective corresponding validation thresholdsin order to verify the quality of the alignment of the inertial unit.

In another example of this first variant, during the comparison, atleast two estimates of the states are combined in order to form anestimated state combination, and an “alignment invalid” signal isactivated when the estimated state combination is greater than or equalto a global threshold. The global value is a predetermined constantcorresponding to a maximum acceptable error value and it is selected asa function of the accuracy expected from the inertial unit.

Such a estimated state combination makes it possible in particular togive preference to monitoring certain states by giving more weight to anestimate of one state than to an estimate of another state. By way ofexample, the estimated state may be combined as a weighted sum of thesquares of at least two state estimates.

In a second variant of the method of the invention, the method may usestate estimates not for the purpose of validating or not validating thealignment, but for correcting the data supplied by the inertial unit sothat, after correction, the data complies with the accuracy expectedfrom the inertial unit, and as a result can be used by the systems ofthe aircraft in which the inertial unit is installed.

Under such circumstances, the additional step of the method in thissecond variant advantageously consists in correcting the data suppliedby the inertial unit so as to make the inertial data usable, with thiscorrection being calculated on the basis of the states estimated by themirror process. For example, each estimate of a state of the mirrorprocess is used as an initial value for a process of estimating errorsof the inertial unit during navigation. This process of estimatingerrors of the inertial unit during navigation is then continuedthroughout the entire duration of the stage of navigation of theinertial unit following the alignment stage. Finally, the errorestimates as maintained in this way are subtracted from the datasupplied by the inertial unit.

Furthermore, in order to obtain a good estimate of the parameters of theinterfering movements of the inertial unit and in order to deduce theresulting errors of the inertial unit accurately, the method of theinvention preferably makes use of movement measurements that arethemselves accurate. By way of example, these movement measurements areadvantageously supplied by a GNSS receiver making use of the phaseincrements of the carrier waves of the signals transited by thesatellites. Specifically, the resulting measurement noise, in particularconcerning velocity measurements, is much smaller than the noise ofmethods conventionally used, such as those based on the derivative ofposition or on observing the Doppler effect on said carriers. Animplementation of this technique is described by way of example in themagazine “Inside GNSS”, in the “GNSS solutions” column, March-April2015, under the title “How does a GNSS receiver estimate velocity?”

For velocity measurements, the accuracy that results from the carrierphase increment technique is of the order of a few millimeters persecond while making use solely of the signals from the satellites. Forposition measurements, accuracy may be of the order of a fewmillimeters, but obtaining such accuracy requires not only signals fromsatellites, but also a stationary ground station communicating with theaircraft.

As a result, the method of the invention enables a central unit that isbeing monitored by the movement sensor to be aligned. The movementmeasurements supplied by the movement sensor are specifically not mixedor combined with the data delivered by the inertial unit.

Consequently, if the movement sensor provides measurements that arecorrupted, that gives rise to a false alarm being issued correspondingto a “moved alignment”, but under no circumstances is the data suppliedby the inertial unit corrupted by those movement measurements.

The present invention also provides a system for analyzing andmonitoring interfering movements of an inertial unit of an aircraftduring an alignment stage. Such a system for analyzing and monitoringinterfering movements of an inertial unit during an alignment stagecomprise a movement sensor for sensing movement of the aircraft andsupplying measurements of movement of the aircraft relative to theground, together with an estimator for estimating a mirror process ofstructure close to the structure of the alignment process of theinertial unit. The estimator has at least one calculator or processorand at least one memory storing in particular calculation instructionsand optionally thresholds for validating the alignment of the inertialunit.

The estimator is configured to perform the above-mentioned method so asto estimate states of the mirror process from observations constitutedby movement measurements supplied by the movement sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its advantages appear in greater detail from thecontext of the following description of implementations given by way ofillustration and with reference to the accompanying figures, in which:

FIG. 1 shows an aircraft having a system for analyzing and monitoringinterfering movements of an inertial unit;

FIG. 2 shows a system for analyzing and monitoring interfering movementsof an inertial unit;

FIG. 3 is a diagram summarizing a method of analyzing and monitoringinterfering movements of an inertial unit;

FIG. 4 shows a model of an alignment process of an inertial unit;

FIG. 5 shows a mirror process; and

FIG. 6 shows measurements of movement of the aircraft.

Elements present in more than one of the figures are given the samereferences in each of them.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a rotary wing aircraft 20. The aircraft 20 has an inertialunit 1 and a system 10 for analyzing and monitoring interferingmovements of the inertial unit 1 during a stage of aligning the inertialunit 1. The system 10 is shown in detail in FIG. 2 and it includes amovement sensor 2 for sensing movement of the aircraft 20 and anestimator 5 having a calculator 3 and a memory 4.

The movement sensor 2 is a GNSS receiver and it supplies accuratemeasurements of movement of the aircraft 20 relative to the ground,which measurements may be velocity or position measurements, thesemeasurements being based by way of example on phase increments ofcarrier waves of signals transmitted by satellites in at least one GNSSsystem. The memory 4 stores calculation instructions and possiblythresholds for validating the alignment of the inertial unit 1. Thecalculator 3 makes use of these calculation instructions, of themovement measurements, and where applicable of the thresholds forvalidating the alignment of the inertial unit 1 in order to perform amethod of analyzing and monitoring interfering movements of the inertialunit 1, as shown in the summary diagram of FIG. 3.

The movement sensor 2 is connected to the estimator 5 in order to supplyit with measurements of the movement of the aircraft 20 relative to theground. The inertial unit 1 is connected to the system 10 in order tosupply it with a start signal t₀ and an end signal t₁ marking the startand the end of the alignment stage.

The method of analyzing and monitoring interfering movements of theinertial unit 1 during an alignment stage comprises two steps.

An acquisition step 110 for acquiring measurements of the movement ofthe aircraft 20 relative to the ground is performed by means of themovement sensor 2 during the stage of aligning the inertial unit 1.

An estimation step 120 of estimating states of a mirror process is alsoperformed from the observations constituted by the movementmeasurements. The model of the mirror process has a structure that isclose to the structure of the model of the alignment process of theinertial unit 1.

The model of the mirror process could equally well be rigorouslyidentical to the model of the alignment process of the inertial unit 1.

During the alignment stage, the alignment process seeks to estimate thevertical direction, by zeroing the two angles of inclination of theinertial unit 1 about North/South and East/West geographical axes, toestimate the direction of North by zeroing the misalignment angle of theinertial unit 1 about the vertical axis, and finally to estimate thecomponents of the velocity of the aircraft 20 relative to the ground. Byway of example, the alignment process of an inertial unit 1 is a systemhaving five states, comprising:

-   -   i) the three angular differences between the axes of the virtual        platform of the inertial unit 1 and the corresponding directions        of the local geographical axes; and    -   ii) the two horizontal components of the velocity of the        inertial unit 1 relative to the ground.

If the inertial unit 1 is genuinely stationary during the alignmentstage, these angular differences and horizontal velocity componentsconverge towards zero values and the inertial unit 1 is then correctlyinitialized.

A block diagram of an example of a model of the process of aligning theinertial unit 1 is shown in FIG. 4. For this alignment process, θ_(n) isthe orientation error about the North/South axis, θ_(e) is theorientation error about the East/West axis, θ_(d) is the orientationerror about a vertical axis, v_(n) is velocity along the North/Southaxis, and v_(e) is the velocity along the East/West axis.

This model of the alignment process takes account of the known latitudeϕ of the aircraft 20 and uses both the modulus of the acceleration ofterrestrial gravity g and a vector representing the angular velocity{right arrow over (Ω)}_(E) of the earth about its axis. Two projectionsΩ_(n), Ω_(d) of this angular velocity vector {right arrow over (Ω)}_(E),respectively onto the North/South axis and onto the vertical axis anddepending on the latitude ϕ are calculated as follows:Ω_(n)=Ω_(E)·cos ϕ and Ω_(d)=Ω_(E)·sin ϕ

In FIG. 4, the symbol ⊗ represents an adder and the symbol

represents an integrator. The acceleration errors γ_(n) along theNorth/South axis and γ_(e) along the East/West axis are also marked.

An estimator of the states of the alignment process may then be a Kalmanfilter having these five states. In other examples of the alignmentprocess, one or more original states may optionally be used. Forexample, the latitude ϕ of the aircraft 20, should it be unknown, mayconstitute an additional state of the alignment process and may then bedetermined by the estimator.

The mirror process model used by the method of the invention may be amodel that is rigorously identical to the alignment process model shownin FIG. 4. Under such circumstances, both models have the same number ofstates and the same matrices defining the relationship between thosestates.

It is also possible for the mirror process model to be simplified orindeed approximated relative to the alignment process model. Forexample, the 24-hour mode associated with rotation of the earth hasspecifically been ignored in order to set up the mirror process shown inthe form of a block diagram in FIG. 5.

When operating in an open loop, the model of this FIG. 5 mirror processgenerates time velocity profiles on its output having the form of twotime polynomials:v _(e)(t)=θ_(n0) ·g·t+V _(e0)andv _(n)(t)=−½·(θ_(d0)·Ω_(n)+θ_(n0)·Ω_(d))·g·t ²+θ_(e0) ·g·t+V _(n0)

From these equations, it can be deduced that the movements having aneffect on the accuracy of the alignment of the inertial unit 1 aremovements consisting in:

-   -   i) a speed ramp along the East/West axis, said ramp being        defined by the coefficients θ_(n0) and V_(e0); and    -   ii) a velocity parabola along the North/South axis, said        parabola being defined by the coefficients θ_(d0), θ_(n0),        θ_(e0), and V_(n0).

By using the movement measurements, which in this example aremeasurements of the speed of the aircraft 20 relative to the groundduring the stage of aligning the inertial unit 1, the method of theinvention makes it possible to identify the coefficients of these twopolynomial functions. FIG. 6 shows these measurements of the velocity ofthe aircraft 20 relative to the ground along the North/South axisobtained during the alignment stage, i.e. between the start t₀ and theend t₁ of the alignment stage, together with a representation of thepolynomial function corresponding to this speed along the North/Southaxis.

The coefficients of these polynomial functions are directly associatedwith the states of the mirror process. The estimation step 120 ofestimating the states of the mirror process consists either inestimating these coefficients for each polynomial function, from whichcoefficients these states are deduced, or else in estimating the statesdirectly. The states of the mirror process can be estimated fromobservations constituted by the movement measurements by using knownmathematical methods such as the non-recursive least squares method orindeed the recursive least squares method, or else by using a Kalmanfilter.

The method of the invention may make use during an additional step 130,140 of these estimated errors for the orientation and the velocities ofthe inertial unit 1 due to the interfering movements during thealignment stage.

In a first variant of the invention, an additional comparison step 130of the method comprises comparing the absolute value of at least oneestimate of the orientation and velocity errors of the inertial unit 1with at least one validation threshold, and activating an “alignmentinvalid” signal if at least one of the thresholds is exceeded.

Activating this “alignment invalid” signal then indicates that theaccuracy of the data supplied by the inertial unit 1 is deemed to beinsufficient, and that the stage of aligning the inertial unit 1 must berestarted, for example, or that the aircraft must be operated in a modethat does not rely on inertial measurements.

In a second variant of the invention, an additional correction step 140of the method consists in correcting the data supplied by the inertialunit 1. This correction 140 makes use of the estimated orientation andvelocity errors of the inertial unit 1 that results from interferingmovements during the alignment stage and that have already beencalculated, in order to improve the data supplied by the inertial unit 1and make that data sufficiently accurate to be usable.

Naturally, the present invention may be subjected to numerous variationsas to its implementation. Although several implementations aredescribed, it will readily be understood that it is not conceivable toidentify exhaustively all possible implementations.

For example, the movement sensor may be arranged outside the system 10and may provide its movement measurements to the system 10 so that thesystem can use them.

Naturally, it is possible to envisage replacing any of the meansdescribed by equivalent means without going beyond the ambit of thepresent invention.

What is claimed is:
 1. A method of analyzing and monitoring interferingmovements of an inertial unit of an aircraft during a stage ofstatically aligning the inertial unit, the method being wherein duringthe stage of statically aligning the inertial unit, the following stepsare performed: an acquisition step for acquiring measurements of themovement of the aircraft relative to the ground by a movement sensor; anestimation step for estimating states of a mirror process of a modelthat is simplified and/or approximated relative to the model of thestatic alignment process of the inertial unit, the states of the mirrorprocess being estimated from observations constituted by the movementmeasurements; and an additional comparison step comprising comparingdata supplied by the inertial unit to detect if interfering movementsare present to cause the supplied data to be erroneous; if the supplieddata is determined to be erroneous, either generate an alignment invalidsignal or correct the supplied data in order to make the supplied datasupplied usable.
 2. The method according to claim 1, wherein validationthresholds for validating the static alignment are defined prior to thestage of statically aligning the inertial unit, each validationthreshold corresponding to a respective one of the estimated states, andthe additional comparison step comprises comparing the absolute value ofat least one estimate of one of the states with at least one validationthreshold.
 3. The method according to claim 2, wherein during theadditional comparison step, the absolute value of at least one estimateof the state is compared with the corresponding validation threshold,and an “alignment invalid” signal is activated whenever the absolutevalue of an estimate of a state is greater than the correspondingvalidation threshold.
 4. The method according to claim 2, wherein duringthe additional comparison step, at least two estimates of the states arecombined in order to form an estimated state combination, and an“alignment invalid” signal is activated when the estimated statecombination is greater than a global threshold.
 5. The method accordingto claim 4, wherein the estimated state combination is equal to aweighted sum of the squares of at least two estimates of the states. 6.The method according to claim 1, wherein the additional comparison stepcomprises an additional correction step that comprises the correctingdata supplied by the inertial unit in order to make the supplied data bythe inertial unit usable, the additional correction step using theestimates of the orientation and velocity errors of the inertial unitresulting from interfering movements during the alignment stage.
 7. Themethod according to claim 6, wherein during the additional correctionstep, the correction is calculated from the estimated states, eachestimated state being used as an initial value for a process ofestimating errors of the inertial unit, the process of estimating errorsbeing maintained throughout the duration of a stage of navigation of theinertial unit following the alignment stage, and the maintained estimateof the errors being subtracted from the data supplied by the inertialunit.
 8. The method according to claim 1, wherein during the acquisitionstep, the measurements of movements of the aircraft relative to theground are measurements of a velocity ({right arrow over (v)}_(g)) ofthe aircraft relative to the ground.
 9. The method according to claim 1,wherein during the acquisition step, the measurements of movement of theaircraft relative to the ground are measurements of a position ({rightarrow over (x)}_(g)) of the aircraft relative to the ground.
 10. Themethod according to claim 1, wherein the estimation step of states ofthe mirror process consist in estimating the coefficients of at leastone polynomial function of time close to the movement measurements. 11.The method according to claim 10, wherein the movement measurements aremeasurements of velocity {right arrow over (v)}_(g) relative to theground, and the model of the mirror process is suitable for generating asecond degree polynomial function of time for the North/South componentof velocity relative to ground, and a first degree polynomial functionof time for the East/West component of velocity relative to the ground.12. The method according to claim 10, wherein the estimation step ofstates of the mirror process is performed by the least squares method inwhich the coefficients for identification are those of the at least onepolynomial function of time.
 13. The method according to claim 1,wherein the estimation step of states of the mirror process consist inKalman filtering for which the states are those of the mirror processand for which the observations are the measurements of movement relativeto the ground.
 14. The method according to claim 1, wherein the movementsensor comprises at least one of a velocity sensor capable of measuringvelocity of the aircraft relative to the ground and a position sensorcapable of measuring position of the aircraft relative to the ground.15. The method according to claim 14, wherein the movement sensorcomprises a GNSS receiver or a Doppler effect radar.
 16. A system foranalyzing and monitoring interfering movements of an inertial unit of anaircraft during a stage of statically aligning the inertial unit, thesystem comprising: a movement sensor for sensing movement of theaircraft and supplying measurements of the movement of the aircraftrelative to the ground; an estimator for estimating a mirror processclose to the static alignment process of the inertial unit, theestimator being provided with at least one calculator and with at leastone memory storing thresholds for validating the static alignment of theinertial unit and calculation instructions, the estimator serving toestimate states of the mirror process having a model that is close tothe model of the static alignment process of the inertial unit, theestimation step of the states of the mirror process being performed onthe basis of observations constituted by measurements of movementsupplied by the movement sensor, wherein data supplied by the inertialunit is used to generate an alignment invalid signal when an estimatedstate combination is greater than or equal to a global threshold andcausing restarting statically aligning the inertial unit.
 17. A methodof analyzing and monitoring interfering movements of an inertial unit ofan aircraft during statically aligning the inertial unit, the methodcomprising: acquiring measurements of the movement of the aircraftrelative to the ground by a movement sensor; estimating states of amirror process of a model that is simplified and/or approximatedrelative to the model of the static alignment process of the inertialunit, the states of the mirror process being estimated from observationsconstituted by the movement measurements; and comparing data supplied bythe inertial unit to a threshold to determine if the supplied data isuseable; if the supplied data is useable, using the supplied data tostatically align the inertial unit, if the supplied data is not useable,correcting the data or restarting statically aligning the inertial unitin order to make the data supplied by the inertial unit usable.
 18. Themethod according to claim 17, wherein if an estimated state combinationis greater than or equal to a global threshold, an alignment invalidsignal is generated.
 19. A method of analyzing and monitoringinterfering movements of an inertial unit of an aircraft during a stageof statically aligning the inertial unit, the method being whereinduring the stage of statically aligning the inertial unit, the followingsteps are performed: acquiring measurements of the movement of theaircraft relative to the ground by a movement sensor; and estimatingstates of a mirror process of a model that is simplified and/orapproximated relative to the model of the static alignment process ofthe inertial unit, the states of the mirror process being estimated fromobservations constituted by the movement measurements; and comparingdata supplied by the inertial unit to detect if interfering movementsare present to cause the supplied data to be erroneous to determine ifthe stage of statically aligning the inertial unit with the datasupplied by the inertial unit should continue, if the supplied data isuseable, statically aligning the inertial unit, if the supplied data isnot useable, making the data supplied by the inertial unit usable.